Ad Hoc Wireless Networks (A Communication-Theoretic Perspective) || Concluding Remarks

Chapter 10

Concluding Remarks

10.1 IntroductionIn this concluding chapter, after introducing a novel communication-theoretic (or physicallayer oriented) perspective in ad hoc wireless networking, we first outline possible extensionsof the theoretical framework. Open problems for the interested reader are presented insection 10.2. Section 10.3 briefly describes the possible architectures for ad hoc wirelessnetworks, i.e. ‘purely’ ad hoc or hybrid ad hoc. In section 10.4, a brief overview of networkapplication architectures is given. In section 10.5, the major standards which will significantlyinfluence the development and diffusion of ad hoc wireless networks are highlighted. Possibleapplications of an ad hoc wireless networking are considered in section 10.6. The chapter isconcluded with a few important observations in section 10.7.

10.2 Extensions of the Theoretical Framework: OpenProblems

10.2.1 Performance of Ad Hoc Wireless Networks: Random VersusUniform Topologies

An ad hoc wireless network, in its pure form, is a wireless network which does not rely onany fixed infrastructure. Without the help of any central controller, nodes self-organize tocommunicate among themselves. Based on mutual cooperation, a message from a source canbe sent to a desired destination several hops away by relying on intermediate nodes to forwardthe message.

The majority of current works in the literature on performance evaluation of ad hocwireless networks rely on computer simulations. This has two potential limitations: (i) itoften takes a long time to obtain statistically accurate results and (ii) simulations mightnot scale well, making it difficult to analyze the performance of large-scale ad hoc wirelessnetworks. Consequently, being able to evaluate the performance of ad hoc wireless networksanalytically or semi-analytically becomes significant. In this book, to gain some fundamentalinsights, in several chapters we have considered a regular network topology. Clearly, this isnot very realistic and, therefore, it is necessary to extend the analytical framework proposedin Chapters 2 and 3 to more realistic ad hoc wireless network scenarios, where the network

Ad Hoc Wireless Networks: A Communication-Theoretic Perspective Ozan K. Tonguz and Gianluigi Ferrari© 2006 John Wiley & Sons, Ltd. ISBN: 0-470-09110-X

250 Chapter 10. Concluding Remarks

1000 2000 3000 4000 5000 6000 7000 80000

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12x 10

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N

CTe

[b−m/s]

Random TopologyGrid Topology

Figure 10.1 Comparison in terms of the effective transport capacity for networks with gridand random topologies. The major network parameters are Pt = 1 µW and Rb = 1 Mb/s.

topology is random. In doing so, one can resort to a semi-analytical approach for performanceevaluation. In particular, one can compare the performance of a network with a well-structured square grid topology with that of a network with a two-dimensional Poissontopology, which can be shown to be ‘the most random’ topology. The network performancecan be studied via two fundamental performance metrics frequently used in this book: biterror rate (BER) and effective transport capacity.

We now show preliminary results of our study on the impact of random topology.In Figure 10.1, the effective transport capacities of networks with grid and random topologiesare shown as functions of the number of nodes N . Only the ideal (no interference) scenariois considered. The coverage area of the network in this case is fixed to A = 1 km2; therefore,increasing the number of nodes corresponds to increasing the node spatial density. The resultsare obtained based on the assumptions that the transmit power is Pt = 1 µW, the data-rateis Rb = 1 Mb/s, and there is free-space loss (γ = 2). The average packet transmission rateis λ = 1 pck/s and the packet length is L = 1000 b/pck. These parameters are typical for alow-power wireless sensor network. In particular, the very low transmit power (Pt = 1 µW)is reasonable because we are assuming only free-space loss (the path loss exponent, denotedas γ , is equal to 2). Should the propagation loss be stronger (γ ≥ 3), then Pt would need tobe significantly increased. From Figure 10.1, it can be observed that as the number of nodes(i.e. the node spatial density) increases, the effective transport capacity increases. However,the effective transport capacity of a network with random topology is significantly lower thanthat of a network with grid topology for low values of the node spatial density. Interestingly,beyond a critical value of the node spatial density, the effective transport capacities for thetwo topologies coincide.

Our results show that the BER degrades significantly when the network has a randomtopology instead of a square grid structure. In addition, our preliminary results show that theend-to-end BER is better if a route consists of many short hops rather than a few long hops.This suggests that a conventional routing protocol which takes the minimum number of hopsas the main criterion for selecting a route may not always be the best strategy, as already

10.2. Extensions of the Theoretical Framework: Open Problems 251

shown in Chapter 9. In addition, the effective transport capacity of a network with randomtopology appears to be significantly lower than that of a network with grid topology. Thisimplies that in order to achieve the same information flow across the network, a networkwith random topology must be much denser than the one with square grid topology, i.e. theremust be a much higher number of nodes deployed over the same surface. This informationis relevant for the design of ad hoc wireless networks such as sensor networks; that is, if thenumber of sensors significantly affects the cost, then a designer may consider placing sensornodes in a well-organized topology, rather than distributing them randomly.

10.2.2 Impact of Clustering on the BER Performance in Ad HocWireless Networks

Although a perfectly regular node spatial distribution, where the nodes are at the vertices of asquare grid, is very useful for understanding the dynamics of multi-hop radio communicationand the impact of physical layer characteristics on the upper layers (see Chapters 2 and 3),it is nonetheless unrealistic. Considering, as an example, the case of a smart dust-like sensornetwork [49], where nodes may be literally thrown over the terrain, it is very likely thatthe final distribution of the nodes will be irregular. This irregularity significantly affects theconnectivity of the network [31].

The performance study of an ad hoc wireless network with random node distribution,and thus random clustering, requires a statistical analysis and usually entails the useof computer simulations [81]. Moreover, the identification of disjoint clusters could beproblematic as well. In order to gain preliminary insights into the impact of clustering onthe performance of multi-hop ad hoc wireless networks, our preliminary analysis of itsresults suggest that one can impose some regularity in the cluster distribution. The obtainednetwork topology, referred to as regularly clustered, can be completely characterized byonly two parameters. The considered topology constraints, although idealistic, lead toa simple parameterized analytical model which compactly allows one to evaluate thenetwork performance. In particular, the BER at the end of a multi-hop communicationroute can be analyzed. Moreover, a meaningful comparison between the performance of aregularly clustered network communication scenario and that of a perfectly regular networkcommunication scenario can be made. Our preliminary results show that a single ‘long’inter-cluster hop can significantly degrade the performance. A simple power control strategyappears to be effective in combating the negative effects of clustering. The proposed approachcan describe many realistic situations, especially for sensor networks. In fact, it is verylikely that these networks will be clustered and that regularity inside each cluster may bedeliberately introduced (e.g. a seismic sensor network, where sensors are placed concentratedin specific regions over a wide surface).

Considering a global circular network area A, in a realistic network communicationscenario, nodes could organize themselves in randomly shaped clusters, shown by theshaded regions in Figure 10.2 (a). An analysis of such a randomly clustered networkcommunication scenario requires a statistical model of the node distribution and involvescomputer simulations. Moreover, it is extremely difficult to model analytically the shapes ofnon-regular clusters. In order to derive a simple analytical model, one can impose a geometricregularity in the cluster structure. In particular, one can assume that: (i) all clusters are circularand have the same dimension and (ii) the centers of the clusters are at the vertices of a squaregrid. This topology is depicted in Figure 10.2 (b) and can be referred to as regularly clustered.One can further assume that inside each cluster the nodes are distributed over a regular grid


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rcl (intra-cluster radius)

Figure 10.2 Clustered ad hoc wireless networks: (a) random and (b) regularly clustered.(Reproduced by permission of © 2004 IEEE.)

– in other words, each cluster is a small-scale version of an ad hoc wireless network withregular topology, as considered in Chapter 2.

A regularly clustered node topology can be simply characterized by the followingdistances (also shown in Figure 10.2 (b)).

• The inter-cluster distance, denoted as di-cl and corresponding to the distance betweenthe centers of two neighboring clusters. This distance is formally defined as

di-cl � rA

i-cl(10.1)

where rA �√

A/π is the radius of the overall circular area A and i-cl ≥ 1 is aparameter which quantifies how many clusters lie over a radius of the global area.

• The radius of a cluster, denoted as rcl, and defined (considering the inter-clusterdistance di-cl as a reference):

rcl � di-cl

cl= rA

i-clcl(10.2)

where the parameter cl ≥ 2 quantifies how small a cluster is compared to the inter-cluster distance.

In Figure 10.3, the performance in a regularly clustered network scenario with N = 103

nodes, one inter-cluster hop and√

N/π − 1 intra-cluster hops is shown as a function of theoverall node spatial density ρS (for a network with regular topology) or cluster node spatialdensity ρcl

S (for a regularly clustered network). The major network parameters are shownin the figure, and we assume an ideal communication scenario without interference. Theperformance strongly depends on the ratio i-cl/cl. In particular, the following commentscan be made considering the cases of large and small values of the ratio i-cl/cl, respectively.


10-3

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10-1

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-2]

10-5

10-4

10-3

10-2

10-1

100

BERroute

Regular

Regularly clustered

i-cl/ cl=1->9

i-cl/ cl=1

i-cl/ cl=9

N=103

fc=2.4 GHz

Ptcl=Pt

i-cl=0.23 W

F=6 dB

Gt=Gr=fl=1

Figure 10.3 BER performance of regularly clustered networks (dashed lines) for N = 103

nodes and one inter-cluster hop. Various regularly clustered geometries (in terms of cl andi−cl) are considered. For comparison, the BER performance of a perfectly regular network(solid line) is also shown.

• Large ratio i-cl/cl. This means that i-cl is large and/or cl is small – recall thatcl ≥ 2. The fact that i-cl is large means that there are many clusters, while thefact that cl is small means that the clusters are close to each other. In this case, theregularly clustered distribution approaches a globally regular distribution. Moreover,an inter-cluster hop is not significantly larger than an intra-cluster hop.

• Small ratio i-cl/cl. This implies that i-cl is small and/or cl is large. The fact thati-cl is small means that there are relatively few clusters, and the fact that cl is largemeans that the clusters are relatively small compared to the global area A, i.e. they arefar apart from each other. In this case, an inter-cluster hop is significantly larger thanan intra-cluster hop, and the performance is thus significantly degraded.

10.2.3 Impact of Receiver Sensitivity on the Performance of Ad HocWireless Networks

One of the important issues in ad hoc wireless networks is the choice of the medium accessscheme. Lack of central authority and constantly changing topology lead to interesting trade-offs and to interactions between physical, medium access control (MAC) and routing layersof the protocol stack. In particular, the impairments introduced by the wireless channelmake the choice of the MAC protocol critical. The use of scheduling mechanisms – such


as time division multiple access (TDMA), frequency division multiple access (FDMA) orcode division multiple access (CDMA) – in large ad hoc wireless networks is quite complex,due to synchronization reasons. Random access then seems a more appealing choice for largead hoc wireless networks. Random access MAC protocols for one-hop wireless networks,such as Aloha and carrier sense multiple access (CSMA), have been studied in great detailin the literature [59, 60]. The use of CSMA in wireless networks leads to well-knownproblems, such as hidden and exposed terminal problems, which are partially solved, forexample, by using multiple access collision avoidance (MACA) MAC protocol [242] andpossible extensions [243]. A lot of research has been done regarding the interaction betweenthe MAC protocol used in the IEEE 802.11 standard [48] and routing in ad hoc wirelessnetworks [244–246]. The impact of unequal carrier sensing and transmitting ranges has beenconsidered in [131, 247–250]. The effect of variations in network size, network density, andtraffic load on the performance of an ad hoc wireless network using the CSMA MAC protocolhas also been studied [82].

In Chapter 3, we have presented two simple MAC protocols, defined as reserve-and-go(RESGO) and reserve-listen-and-go (RESLIGO), respectively. In particular, the RESLIGOMAC protocol is characterized by the fact that a node, after reserving a multi-hop route to itsdestination, senses the channel before transmitting: if no transmission is going on, then thesource node starts transmitting, i.e. it activates the route (it ‘goes’). In this case, the receiversensitivity at each node is assumed to be infinite: since all nodes in the network can ‘hear’each other, in Chapter 3 it is shown that, for most practical scenarios, there is only a singleactive multi-hop route in the network at a time.

It is of interest to extend the RESLIGO MAC protocol to account for limited receiversensitivity (LRS), which reflects a practical scenario. In [132], this MAC protocol is referredto as LRS-RESLIGO. Further research is needed to analyze the impact of its use on theperformance of an ad hoc wireless network in terms of BER and effective transport capacity.Our preliminary results show that for large values of the receiver sensitivity the LRS-RESLIGO MAC protocol reduces to the RESGO MAC protocol, whereas for sufficientlylow values of the receiver sensitivity it reduces to the RESLIGO MAC protocol. For a givenvalue of the traffic load, there exist optimal values of transmission data-rate and receiversensitivity for the maximization of the effective transport capacity, i.e. for the maximizationof information transfer across the network [132]. Hence, this provides valuable directions fornetwork optimization.

10.2.4 Spectral Efficiency–Connectivity Tradeoff in Ad Hoc WirelessNetworks

The concept of effective transport capacity has been introduced in Chapter 3 to quantifythe actual bandwidth–distance product supported by the network. A simple and intuitivederivation of an expression for this quantity, in the case of reservation-based ad hoc wirelessnetwork communications with disjoint routes, has been proposed. This expression explicitlyshows how the effective transport capacity is related to the network connectivity level and tothe traffic load generated by the nodes.

The analysis presented in Chapter 3 is limited to binary modulations. One can, how-ever, investigate the impact of the modulation format on the effective transport capacity,considering various channel models. In order to transmit large quantities of data, it seemsappealing to make use of spectrally efficient modulations. While the impact of differentmodulation formats is well understood for single-link communications, its impact in the


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BPSKQPSK NaturaleQPSK Gray8-PSK16-PSK4-PAM Naturale4-PAM Gray16-QAM64-QAMDBPSKMSKA A

N=1000fc=2,4 GHz

Pt=10-7

W

A=106 m

2

BERroute

=10-3

F=6 dB

max

Figure 10.4 CTe (maximized with respect to λL) versus the transmission bandwidth B

for several modulation formats in an ideal (without interference communication scenario).(Reproduced by permission of © 2004 IEEE.)

case of multi-hop wireless networks is not. Considering a simple reservation-based ad hocwireless network communication scenario, one should try to answer the following question:what is the relationship between spectral efficiency of the modulation format and networkconnectivity?

The trade-off between spectral efficiency and network connectivity can be investigated,considering a number of linear modulation formats and simple frequency modulation formats.In Figure 10.4, preliminary results, in terms of maximum effective transport capacity versustransmission bandwidth B, are shown for several linear modulation formats. From the resultsin Figure 10.4, one can observe that the highest possible effective transport capacity isobtained with binary phase shift keying (BPSK) and quaternary phase shift keying (QPSK)with Gray coding. In other words, the highest effective transport capacity is obtained bythe modulation formats with the highest possible energy efficiency. However, this comes atthe expense of the transmission bandwidth, which has to be significantly increased. On theother end, for small transmission bandwidth, the highest possible effective transport capacityis obtained with highly spectrally efficient modulation formats, such as 64-ary quadratureamplitude modulation (64-QAM).


10.2.5 MIMO-OFDM Wireless CommunicationsOrthogonal frequency division multiplexing (OFDM) has become a popular technique fortransmission of signals over wireless channels [251,252]. OFDM has been adopted in severalwireless standards such as digital audio broadcasting (DAB), digital video broadcasting(DVB-T), the IEEE 802.11a local area network (LAN) standard and the IEEE 802.16ametropolitan area network (MAN) standard. OFDM is also being pursued for dedicated short-range communications (DSRC) for road side to vehicle communications and as a potentialcandidate for fourth-generation (4G) mobile wireless systems.

OFDM converts a frequency selective channel into a parallel collection of frequencyflat subchannels. The subcarriers have the minimum frequency separation required tomaintain orthogonality of their corresponding time domain waveforms, yet the signalspectra corresponding to the different subcarriers overlap in frequency. Hence, the availablebandwidth is used very efficiently. If knowledge of the channel is available at the transmitter,then the OFDM transmitter can adapt its signaling strategy to match the channel. Due tothe fact that OFDM uses a large collection of narrowly spaced subchannels, these adaptivestrategies can approach the ideal water pouring capacity of frequency-selective channels.In practice, this is achieved using adaptive bit loading techniques, where different sizedconstellations are transmitted on the subcarriers. OFDM may be combined with antennaarrays at the transmitter and receiver to increase the diversity gain and/or to enhance thesystem capacity on time-variant and frequency-selective channels, resulting in a multiple-input multiple-output (MIMO) configuration.

While MIMO-OFDM techniques might be hard to apply in harsh military applications,in other sensor applications where mobility of nodes is absent and nodes might have enoughbattery power, such techniques could be plausible. Further research is needed to incorporatethe MIMO-OFDM techniques into the communication-theoretic framework developed in thisbook.

10.2.6 Smart Antennas and Directional AntennasThe cost of the MIMO-OFDM system is largely driven by the number of transmit andreceive chains. For example, each receive chain includes frequency conversion, intermediatefrequency (IF) filtering and analog–digital (A-D) conversion. The circuits performing thesefunctions must be replicated L times if a MIMO receiver has L receive branches. However,it is possible to have the spatial diversity and interference suppression benefits of manymore antennas than full receiver chains through the use of antenna or beam selection. Thesetechniques (also known as antenna selection or beam selection) can yield a signal-to-noiseratio (SNR) advantage in scenarios when there is no interference and a signal-to-interference(SIR) advantage in a scenario with interference.

In addition, multicast applications might necessitate the use of smart antennas or direc-tional antennas. Further research is needed to see how such techniques can be incorporatedinto the communication-theoretic framework developed in this book.

10.3 Network ArchitecturesWhile in this book our main focus was on ad hoc wireless networks with flat architectures,there are several very interesting applications of sensor networks and ad hoc wirelessnetworks which have hierarchical architectures. Categorically speaking, there are two majorarchitectures of interest in different applications of ad hoc networks:

10.4. Network Application Architectures 257

A. ‘pure’ ad hoc networks: ‘flat’ architectures;

B. hybrid ad hoc networks: hierarchical architecture.

While most military applications of sensor networks and ad hoc wireless networks willprobably be implemented with networks having ‘flat’ architectures, most of the commercialapplications of sensor networks (such as environmental sensor networks, disaster reliefnetworks, biomedical sensor networks that could be implemented or worn by human beings)and ad hoc wireless networks (e.g. car-based ad hoc wireless networks, such as integratedcellular and ad hoc relay, iCAR systems, [135]) will be hybrid networks in that they willutilize and/or rely on the existing infrastructure and, therefore, will have a hybrid architecture.

It remains to be seen whether and to what extent, the communication-theoretic frameworkdeveloped in this book is also applicable to sensor networks and ad hoc wireless networkswith hierarchical architecture (i.e. hybrid ad hoc wireless networks that also utilize theexisting telecommunications or wireless infrastructure). At a first glance, it appears that mostof the developed framework may also be useful for hybrid ad hoc wireless networks as well.However, further research is needed to see if this is indeed the case.

10.4 Network Application ArchitecturesWhen building a new network application, one first needs to decide on the application’sarchitecture. We remind the reader that an application’s architecture is distinctly differentfrom the network architecture (e.g. the five-layer TCP/IP protocol stack of the Internetarchitecture). From the application developer’s perspective, the network architecture is fixedand provides a set of services to applications. The application architecture, on the otherhand, is designed by the application developer and dictates how the application is organizedover the various end systems. In choosing the application architecture, an applicationdeveloper will likely draw on one of the predominant architectures used in modern networkapplications [167]:

A. the client–server architecture;

B. the peer-to-peer (P2P) architecture;

C. a hybrid of the client–server and P2P architectures.

In a client–server architecture, there is an always-on host, called the server, whichservices requests from many other hosts, called clients. A classic example is the Webapplication for which an always-on Web server services requests from browsers runningon client hosts. When a Web server receives a request for an object from a client host,it responds by sending the requested object to the client host. Note that with the client–server architecture, clients do not communicate with each other; for example, in the Webapplication, two browsers do not directly communicate. Another characteristic of the client–server architecture is that the server has a fixed, well-known address, and because the server isalways on, a client can always contact the server by sending a packet to the server’s address.Some of the better-known applications with a client–server architecture include the Web, filetransfer, remote login and e-mail.

In a pure P2P architecture, there is not an always-on server at the center of the application.Instead, arbitrary pairs of hosts, called peers, communicate directly with each other. Because


Table 10.1 Summary of 802.11 standards

Standard Frequency range Data Rate

802.11b 2.4–2.5 GHz ≤ 11 Mb/s802.11a 5.1–5.8 GHz ≤ 54 Mb/s802.11g 2.4–2.485 GHz ≤ 54 Mb/s

the peers communicate without passing through some special server, the architecture is calledpeer-to-peer. In the P2P architecture, none of the participating hosts is required to be alwayson; in addition, a participating host may change its IP address each time it comes on. A niceexample of an application that has a pure P2P architecture is Gnutella, an open-source P2Pfile-sharing application. In Gnutella, any host can request files, send files, query to find wherea file is located, respond to queries and forward queries. One of the greatest strengths of theP2P architecture is its scalability. Thus, in principle, P2P file sharing is intrinsically scalable –each additional peer not only increases demand but also increases service capacity. In today’sInternet, P2P file-sharing traffic accounts for a major fraction of all traffic.

Client–server and P2P are two common architectures for network applications. However,many applications are organized as hybrids of the client–server and P2P architectures. Onesuch example is the now dysfunctional Napster, which was the first of the popular MP3 file-sharing applications. Napster is P2P in the sense that MP3 files are exchanged directly amongpeers, without passing through dedicated, always-on servers; but Napster is also client–serverin the sense that a peer queries a central server to determine which currently-up peers have adesired MP3 file. Another application that uses a hybrid architecture is instant messaging.

It is fair to say that the theoretical framework developed in this book is valid for P2Papplication architectures. Hence, it remains to be seen whether for client–server or hybridnetwork applications the same framework can be applied. Clearly, further research is requiredto understand and establish this.

10.5 StandardsIn this section, we aim to sensitize the reader to several open issues which need to beaddressed over the next several years. While our treatment in this book has focused more onrandom access type MAC protocols (RESGO, RESLIGO, etc.), this, by no means, precludesthe use of other types of MAC protocols such as scheduling, TDMA, CDMA, etc. Clearly,the size of the network, the coverage area, the battery power, the network lifetime required(longevity), the desired QoS (goodput and delay) and the transport capacity targeted inspecific applications might ultimately determine the type of MAC protocol one needs toemploy in that specific application. Preliminary considerations show that in larger ad hocnetworks with thousands of nodes use of scheduling in the form of TDMA may be difficult,if not impossible. If the network, however, is much smaller (of the order of a few hundrednodes) and the application at hand is a hybrid architecture utilizing some of the existingtelecommunications infrastructure, then scheduling (e.g. TDMA) might be plausible.

Based on these considerations, we highlight below some immediate standards which mayuse ad hoc communication in one form or another [167].

10.5. Standards 259

A. IEEE 802.11 Wireless Local Area Networks (WLANs): Wi-FiOver the last 4 to 5 years, we have witnessed pervasive use of wireless LANs in theworkplace, the home, university campuses, cafés, airports, etc. [167].

There are several 802.11 standards for WLAN technology, including 802.11b, 802.11aand 802.11g. Table 10.1 summarizes the main characteristics of these standards.Among these standards, the 802.11b WLANs are by far the most prevalent. However,802.11a and 802.11g products are also widely available, and these higher-speedWLANs should enjoy significant deployment in coming years.

The fundamental components of the network architecture of 802.11 WLANs are abunch of base stations (known as access points, APs, in 802.11 technology) connectedvia an Ethernet infrastructure that interconnects the APs and a router, and severalwireless stations (or nodes) getting service via these APs.

The 802.11 MAC protocol currently in use is CSMA with collision avoidance(CSMA/CA). As with Ethernet’s CSMA with collision detection (CSMA/CD), the‘CSMA’ in CSMA/CA stands for ‘carrier sense multiple access’, meaning that eachstation senses the channel before transmitting, and refrains from transmitting whenthe channel is sensed busy. Although both Ethernet and 802.11 use carrier-senserandom access, the two MAC protocols have important differences. First, insteadof using collision detection, 802.11 (unlike Ethernet) uses a link-layer acknowledg-ment/retransmission (automatic repeat request, ARQ) scheme.

In the case of Ethernet, a node listens to the channel as it transmits. If, whiletransmitting, it detects that another station is also transmitting, it aborts its transmissionand tries to transmit again after waiting a small, random amount of time. Unlikethe 802.3 Ethernet protocol, the 802.11 MAC protocol does not implement collisiondetection. There are two important reasons for this.

• The ability to detect collisions requires the ability to send (the station’s ownsignal) and receive (to determine whether another station is also transmitting)at the same time. Because the strength of the received signal is typically verysmall compared to the strength of transmitted signal at the 802.11 adapter, it iscostly to build hardware that can detect a collision.

• More importantly, even if the adapter could transmit and listen at the same time(and presumably abort transmission when it senses a busy channel), the adapterwould still not be able to detect all collisions due to the hidden terminal problemand fading.

Because 802.11 WLANs do not use collision detection, once a station begins totransmit a frame, it transmits the frame in its entirety; that is, once a station gets started,there is no turning back. As one might expect, transmitting entire frames (particularlylong frames) when collisions are prevalent can significantly degrade the performance ofa MAC protocol. In order to reduce the likelihood of collisions, 802.11 employs severalcollision avoidance techniques. For example, the 802.11 MAC protocol also includes anifty (but optional) reservation scheme, in the form of request to send (RTS) and clearto send (CTS), that helps avoid collisions even in the presence of hidden terminals.

It is conceivable to think that 802.11 WLANs could be used in ad hoc mode as well. It isnot clear whether the communication-theoretic framework developed in this book can


T1+Level Service Enterprise

Fractional T1 for Small Business

Internet Backbone

Backhaul for Hospitals

Telco Core Network or

Private (Fibre) Network

Residential and SoHo DSL

Developing Countries

Green Field Deployments

e.g., Africa

802.11

802.11802.11

802.16

Always Best ConnectedBackhaul

Figure 10.5 Typical scenario for WiMAX-based wireless network.

also be directly applied to 802.11 WLANs operating in ad hoc mode. Further researchis needed to determine this and also the type of modifications required in the frameworkto handle this application.

B. IEEE 802.16 Standard: WiMAXThe WiMAX standard first appeared in 2002, in order to define a specification forwireless metropolitan area networks (WMANs). In other words, one of the defininggoals of this standard was to make broadband wireless access a reality and a viablealternative to last mile technology based on the use of fiber optic or cable connections.This standard was originally developed for point-to-multipoint scenarios, wherevarious subsidiary stations (SSs) connect to a common base station (BS). In particular,this standard aims at providing a ‘flexible, cost-effective, standards-based means offilling existing gaps in broadband coverage, and creating new forms of broadbandservices not envisioned in a “wired” world’. A typical scenario envisioned in [253]is depicted in Figure 10.5.

Therefore, it becomes clear that a WiMAX-based wireless network will consist ofdifferent users (with different needs) trying to connect to a wired backhaul through acommon BS. In order to support high data-rate communications, the standard considersOFDM technology at the physical layer. Moreover, having started as a point-to-multipoint wireless network communication scenario, scheduling, in terms of TDMA,is considered as the basis of the IEEE 802.16 MAC protocol.

One of the major goals of the WiMAX standard is that of making different WLANsinteroperable. This direction comes from the fact that WLANs are finding increasing

10.5. Standards 261

802.11

802.11802.11

802.16

802.11

802.11802.11

802.16

802.11

802.11802.11

802.16

802.11

802.11802.11

802.16

Always Best Connected

Figure 10.6 A possible scenario for a WiMAX/mesh-based wireless network.

applications in stand-alone mode. In fact, the presence of hot spots is becoming morecommon (e.g. a hot spot in an airport or in a café). It is therefore not difficult to envisionthat these hot spots will possibly be connected to each other. For example, people havealready started talking about a US-wide Wi-Fi coverage [254], and the way to make ita reality is probably to start creating WiMAX-based aggregated WLANs.

We envision the possibility of extending the WiMAX concept by combining it with themore general concept of mesh networks, a concept typically used to refer to networkscreated by the union (or combination) of heterogenous networks. In other words, it isenvisioned that the AP of a WLAN, rather than connecting directly to the BS of theWiMAX architecture, could transmit to another AP, which, in turn, could relay thismessage to the BS. This scenario is shown in Figure 10.6.

The practical scenario in Figure 10.6 can immediately be associated with thelogical scenario depicted in Figure 10.7. In other words, the key idea is that an AP(corresponding to a particular SS), rather than accessing the BS directly, chooses amulti-hop transmission through neighboring SSs. At this point, one can identify thenetwork shown in Figure 10.7 as a particular instance of the ad hoc wireless networkingscenarios considered in our framework. More precisely, since scheduling is going to beused at the base station, it is possible to assign different carrier frequencies (in theconsidered OFDM-based communication scheme) to the different multi-hop routes.

In other words, by associating orthogonal signals to the different multi-hop routes andpossibly using time division multiplexing (TDM), it follows that the multiple access


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Source 1

Source 2

Destination

Figure 10.7 Logical equivalent scenario to that in Figure 10.6.

interference is going to be negligible. In other words, use of scheduling leads toefficient interference reduction. This allows one to immediately apply several of theinsights gained with our theoretical framework for an ideal network communicationscenario, where there is no interference. We are currently pursuing this researchdirection.

C. TCP/IPIt is well known that the TCP/IP protocol was mainly developed for wired networks andit has major problems with wireless networks. Indeed, this issue has been addressed byseveral researchers and several reasonable solutions (such as SNOOP-TCP, Split-TCP,etc.) have been proposed [255].

It is important to understand that the communication-theoretic framework developed inthis book did not particularly consider the capabilities and limitations of the IP protocoland how the application of TCP or UDP would fare in a challenging environment likead hoc wireless networks. Further research is needed to understand the implications ofusing TCP or UDP on top of IP protocol in such an environment. While it is clear thatall the problems of using TCP/IP over wireless links (e.g. in a cellular environment)will also be present in ad hoc wireless networks, other unique characteristics of ad hocnetworks (such as limited battery power, mobility, etc.) may exacerbate some of thewell-known problems of using TCP/IP over wireless links.

Intuitively, one expects to see that such problems might be more severe in ad hocwireless networks with a flat architecture than hybrid ad hoc wireless networks thathave a hierarchical architecture. Again, the jury is out on these critical issues. It isinteresting to observe that while for hybrid ad hoc wireless networks it is almost clearthat the use of TCP/IP may be mandatory, this is less clear for ‘pure’ ad hoc networksor sensor networks that are supposed to function in isolation and without interactingwith any kind of telecommunication infrastructure.

Further research is needed to clarify these important issues which might be crucial forwell-engineered systems.

10.6. Applications 263

Delay

Security

Fairness

...

?

Connectivity

Throughput BER/PER

Network Lifetime(Longevity)

Capacity

Transport

Figure 10.8 ‘Performance’ of ad hoc wireless networks.

10.6 ApplicationsAd hoc wireless networks have the potential to change the society in fundamental ways.While in the last 5 years the military applications of ad hoc wireless networks was theprincipal motivation behind much of the research reported in the literature, it is now clear thatthis area holds the promise of improving human life in both developed and underdevelopedcountries of the world. Among the key commercial/civilian applications one can count theuse of sensor networks/ad hoc wireless networks for:

A. disaster relief (e.g. after an earthquake, tsunami, or 9/11 type of events) [256];

B. environmental applications (e.g. for checking the pollution of air, underground water,etc.) [257];

C. biomedical applications (e.g. placing nanometer-scale sensor integrated circuits intohuman beings for early detection of major diseases such as cancer, multiple sclerosis,Parkinson disease, etc., in addition to better managing diseases such as diabetes) [258];

D. car-based applications (today’s cars are probably the best equipped – in terms ofcomputational processing power – mobile nodes that one could hope for. Creating anon-demand overlay ad hoc network of these cars on a given cellular infrastructure canalleviate the ‘hot spot’ problem [135,139] (such as iCAR) as well as enabling numerousnew applications that could improve the lives of commuters in large cities around theglobe).

It would be interesting to see how one can apply the theoretical framework developed inthis book to cope with these various applications.


10.7 ConclusionsThis book attempts to provide a new communication-theoretic perspective to the study ofad hoc wireless networks. The main emphasis in on taking the capabilities and limitationsof the physical layer into consideration in designing such networks. In addition, the strongcoupling between physical, MAC and network layers is examined and it is shown that across-layer design approach is clearly the optimum approach in designing ad hoc networkswith optimum performance.

Our results indicate that the ‘performance’ of ad hoc wireless networks is a non-trivialconcept in that it entails very different, and sometimes conflicting, metrics and requirements.Perhaps, one can visualize the performance of ad hoc wireless networks in an n-dimensionalvector space, as shown in Figure 10.8.

The main message we would like to convey to the reader is the fact that to maximize(or optimize) all of these performance metrics is not only impossible (because they areconflicting requirements) but also unnecessary. Any given application of sensor networks orad hoc wireless networks will clearly require a few of these metrics to be optimized while theothers must satisfy minimum requirements. It is thus clear that it is of paramount importanceto formulate the key objectives, for the application at hand, that one is trying to maximize(or minimize). In this manner, the dimensionality of the cross-layer optimization problemcan perhaps be reduced to a manageable level.

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Ad Hoc Wireless Networks (A Communication-Theoretic Perspective) || Concluding Remarks